In a conventional cellular communication system including a plurality of wireless transmit/receive units (WTRUs), (i.e., user equipments (UEs), radios, mobile platforms, handsets), and a network, the WTRUs are required to measure signals from different cells in order to find base stations with the strongest signal and report them to the network for facilitating handover (connected mode) and cell reselection (idle mode). When a new base station with a stronger signal than a current serving base station is measured, the network will instruct the respective WTRU to handover to the new base station. These measurements are typically performed on common channels, such as the pilot channel in Code Division Multiple Access (CDMA) networks and the Broadcast Control Channel (BCCH) in Global System for Mobile communications (GSM) networks.
Multi-mode WTRUs support more than one Radio Access Technology (RAT). For example, a multi-mode WTRU may include support for Universal Terrestrial Radio—Frequency Division Duplex (UTRA-FDD) CDMA and GSM. Usually, multi-mode WTRUs are required to support handover between different RATs. Such handovers are referred to as inter-RAT handovers.
A problem arises for the case of multi-mode WTRUs in which one of the modes requires constant transmissions. For example, during an active UTRA-FDD CDMA call, i.e., when in “connected mode”, the WTRUs continuously transmit and receive in different frequency bands. In order to make a measurement for an inter-RAT handover, such as to a GSM or Time Division Synchronous Code Division Multiple Access (TD-SCDMA) network, the WTRU must stop transmitting during the measurement period. Otherwise, emissions from the CDMA transmitter will interfere with the receiver belonging to the other RAT.
FIG. 1 illustrates the interference problem described above. A frequency spectrum 100 includes a UTRA-FDD signal in the 1920–1980 MHz band 105 having spurious emissions 110 that fall within the GSM Digital Cellular System (DCS) 1800 receive band 115 from 1805–1880 MHz. The UTRA-FDD signal 105 itself acts as a large out-of-band blocker for the GSM receiver and desensitizes the receiver if not attenuated. Known practical filter solutions provide some attenuation of the UTRA-FDD signal, but the small frequency spacing and cost/size constraints results in inadequate filtering which does not affect the level of the spurious transmission from the UTRA-FDD transmitter.
Networks that require continuous transmissions using multi-mode operation avoid this problem by allowing the WTRU to create gaps in its transmissions. During the gap period, the WTRU stops transmitting to the serving cell. Instead, the WTRU receives signals from neighboring cells belonging to a different system. For example, by implementing a compressed mode in UTRA-FDD, the WTRU is able to take advantage of the gaps in transmissions to make measurements on GSM cells.
FIG. 2 illustrates an example of a conventional WTRU 200 which implements a compressed mode operation. The WTRU 200 includes an antenna 205, a system switch 210, a duplexer 215, a universal mobile telecommunications system (UMTS) receiver (Rx) 220, a UMTS transmitter (Tx) 225, a transmit/receive switch 230, a bandpass filter 235, a GSM Rx 240, and a GSM Tx 245. The system switch 210 is used to selectively connect the antenna 205 to one of a UMTS (e.g., UTRA-FDD) radio or a GSM radio. By implementing the compressed mode, cell capacity is reduced because communications which occur between the WTRU 200 and the network is interrupted during the above-mentioned transmission gaps. A multi-mode radio that simultaneously receives signals from a UTRA-FDD cell and a Time Division Multiple Access (TDMA) cell to avoid such an interruption in communications is desired. Furthermore, a multi-mode radio that allows the GSM Rx 240 to receive signals without interference from the UMTS Tx 225 is also desired.